Concentration and coating processes

ABSTRACT

An electrolytic bath is concentrated in situ by a process of electrodecantation during the principal electrolytic process for which the bath is employed, e.g. a coating process, utilizing in the electrodecantation process at least some of the electric current which is used in the principal electroytic process. Alternatively, the composition may be concentrated in the electrolytic bath in a separate stage when the principal electrolytic process is not taking place, but there is then used at least one electrode which is also used in the principal electrolytic process.

United States Patent [191 Cooke Feb. 11, 1975 CONCENTRATION AND COATING PROCESSES [75] Inventor: Brian Alfred Cooke, Knotty Green,

England [73] Assignee: Imperial Chemical Industries Limited, London, England [22] Filed: Dec. 17, 1973 [21 App]. No.: 425,289

Related US. Application Data [63] Continuation of Ser. No. 163,378, July 16, 197i,

abandoned.

[30] Foreign Application Priority Data July 22, I970 Great Britain 35516/70 [52] U.S. Cl. 204/181, 204/299 [51] Int. Cl. B0lk 5/02 [58] Field of Search 204/181 [56] References Cited UNITED STATES PATENTS 3,419,488 12/1968 Cooke 204/l8l Lohr 204/181 Madejczyk 204/18 1 Primary ExaminerHoward S. Williams Attorney, Agent, or FirmCushman, Darby & Cushman [57] ABSTRACT trolytic process is not taking place, but there is then used at least one electrode which is also used in the principal electrolytic process.

8 Claims, 3 Drawing Figures CONCENTRATION AND COATING PROCESSES This is a continuation, of application Ser. No. 163,378 filed July 16, 1971, now abandoned.

BACKGROUND OF THE INVENTION a. Field This invention relates to processes of concentrating liquid compositions by electrodecantation, to processes of concentrating and concurrently coating conductive articles from such compositions and to apparatus for use in the processes.

Liquid compositions which comprise ionised material, counterion and a continuous medium may contain continuous medium in excess of that desired in an electrolytic process in which they are, or are to be, employed. Alternatively, it may be desired to separate continuous medium from an ionised material produced in an electrolytic process. Typical such electrolytic pro cesses are respectively those of coating conductive articles and of producing chemicals by electrolysis. Hitherto, it has been frequently necessary to discard the compositions or alternatively to remove continuous medium by an evaporative process or .by the use of plant of high capital cost.

Electrolytic coating processes include (a) electroplating with metals and (b) the electrodeposition of organic film-forming materials, particularly those organic materials which contain acidic or basic groups and which are dispersed in an electrolytic coating bath in the presence of a neutraliser. As coating proceeds in these processes it becomes necessary to add some replenishment coating material to the coating bath in which conductive articles are being coated in order to compensate for the material deposited on the articles. Normally, the replenishment material will be dispersed in a continuous medium which is similar to that in the coating bath so that as coating proceeds, unless continuous medium is otherwise removed, there is an accumulation of continuous medium in the bath.

It is also necessary, as coating material is deposited, to ma ntain optimum conditions in the coating bath. In the case of acidic or basic materials dispersed in the bath in the presence of a neutraliser it is particularly necessary to maintain the pH of the coating bath within a narrow range by the control of neutraliser content.

Articles coated in a coating bath by an electrolytic process emerge from the bath with an electrodeposited coating and an overlying dip coating. It is normally preferred to remove the dip coating by rinsing with water. In current practice, an article coated by electrodeposition with an organic film-forming material may be rinsed with water at a location removed from the coating bath and the rinsings then treated as effluent. There is, however, a serious economic disadvantage due to the loss of coating constituents which are carried away in the rinsings.

It has been proposed to rinse coated articles above the coating bath so that the rinsings fall directly into the bath or, alternatively, rinsings from a rinsing stage removed from the bath may be returned to the bath. Since the rinsings dilute the contents of the coating bath to an undesirable extent, howeveer, and in order to maintain optimum coating conditions, it is necessary to remove the excess continuous medium.

b. Prior Art Although there have been certain proposals for effecting the removal of excess continuous medium from liquid compositions, they are not entirely satisfactory and they generally entail the use of mechanical or hydraulic equipment external of the bath which is liable to failure and is of high capital cost. Some methods require high pressures, and those methods which require the separation of continuous medium by passage through a membrane under pressure demand a membrane of precisely controlled composition and structure. Moreover, these membranes may have a short operational life, for example because of blockage or chemical degradation.

SUMMARY OF THE INVENTION We have now found that a liquid composition of which an electrolytic bath is comprised can be concentrated in situ by a process of electrodecantation during the principal electrolytic process for which the bath is employed, e.g., a coating process, utilising in the electrodecantation process at least some of the electric current which is used in the principal electrolytic process. Alternatively, the composition may be concentrated in the electrolytic bath in a separate stage when the principal electrolytic process is not taking place, but there is then used at least one electrode which is also used in the principal electrolytic process.

According to the invention we provide a process of treating an electrolytic coating bath which contains a main liquid composition comprising continuous liquid medium, ionised material and counterion, the bath having at least one electrode which is used to effect a principal electrolytic process, wherein the main composition is concentrated by introducing at least a portion thereof into a chamber defined in the bath at least in part by two opposed barriers of material substantially impermeable to ionised material but permeable to counterion and passing an electric current between the said electrode and another electrode through the barriers and across the chamber, the components of the introduced composition which accumulate in the region of the barrier surfaces as a result of the passage of electric current being separated and removed from the chamber and a composition enriched in ionised material returned to the main composition.

DETAILED DESCRIPTION OF INVENTION By a principal electrolytic process we mean an electrolytic process, other than an electrodecantation process, which is conducted in a bath of ionised material and from which a useful product results, for example an electrolytic coating process from which results a coated product, or a process of producing chemicals electrolytically.

By an ionised material we mean a material bearing an ionic charge or ionic charges which, together with counterion, play(s) the major part in the principal electrolytic process. In an electrolytic coating process the ionised material is a coating material, for example a metal ion or an organic film-forming material containing ionised acidic or basic groups, whic his deposited on an article as one electrode. The counterion is attracted to the other electrode and when, for example, the ionised material is a film-forming polymeric material containing acid groups, the counterion is a base ion. Alternatively, for example, the ionised material can be the starting material for or the product of an electrolytic process such as anodic oxidation, cathodic reduction, a coupling reaction or double decomposition.

The invention is particularly applicable to the concentration of electrolytic baths which contain a liquid coating composition comprising an ionised coating material; and more particularly a composition in which the ionised coating material in a film-forming material containing acidic or basic groups, this material being dispersed in an aqueous continuous medium in the presence of a neutraliser. This aspect of the invention is considered in more detail hereafter.

In one form of the process in which the electric current is passedjbetween electrodes used to effect a principal electrolytic process, it is a major advantage that an electric current can be utilised to effect concentration of the bath contents by electrodecantation within an electrolytic bath (i.e., removal of excess continuous medium) concurrently with the production of a useful product by the principal electrolytic process. It is, therefore, unnecessary to interrupt a continuous process or to remove quantities of the bath contents for external treatment in order to remove excess continuous medium. Moreover, when using the same electric current the rate of removal of continuous medium can be directly related to the rate of the principal electrolytic process, which rate may itself in large part determine the extent to which continous medium will accumulate in the bath.

In an alternative form of the process, in which the electric current is passed between electrodes only one of which is used to effect a principal electrolytic process, it is a major advantage that the electrolytic bath contents can be concentrated in situ during a pause in an otherwise continuous process or during an interval in a non-continuous process. In such a case it is, of course, necessary that when electric current is passed between the electrodes the principal electrolytic process does not occur. In the case, for example, of an electrolytic coating process in which the ionised material is a film-forming polymer containing acidic or basic groups it is necessary to prevent deposition of the polymer on the electrode to which it is attracted since otherwise, if an insulating coating of polymer is produced on that electrode the flow of current will cease. This may be achieved, for example, by shielding the electrode with a diaphragm to which the polymer either will not adhere or will only loosely adhere, or alternatively with a diaphragm which permits an ionic flux incapable of precipitating coating material from the bath.

Although the chamber in which electrodecantation is effected may be situated in any convenient position in the electrolytic bath, so that an electric current which is passed between the electrodes utilised in the principal electrolytic process, or alternatively between at least one of the electrodes used in the principal electrolytic process and another electrode, also passes through the barriers and across the chamber, it is preferred that the chamber is associated with that electrode (the counter electrode) to which the counterions are attracted when an electric current is passed through the electrolytic bath. Preferably the chamber and the counter electrode are incorporated into an electrode assembly and according to a further feature of the invention an electrode assembly suitable for use in the process described above comprises an electrode and two opposed barriers spaced from the electrode which, at least in part, define a chamber remote from the electrode, said barriers being impermeable to the ionised material but permeable to the counterion.

Preferably, when in use, the opposed barriers are substantially vertical and planar and spaced so that their planes are in substantially parallel relationship. The chamber may be further defined by walls, for example side walls, which may or may not be permeable to counterion but which are impermeable to the ionised material.

By a chamber we mean a space (which is located within the electrolytic bath during the process) which may be incompletely or completely enclosed. When the chamber projects above the surface of the electrolytic bath, for example, it may be open to the atmosphere or it may be open at the bottom, and it is at least in part defined by opposed barriers of, material substantially impermeable to ionised material but permeable to counterion. The chamber may be further defined by barriers or walls which serve to further enclose the chamber. Provision must, of course, be made for the introduction to and exit from the chamber respectively of the electrolytic bath composition and the separated components of the introduced composition. This may be achieved through entry and exit ports in the barriers or walls which define the chamber. In one preferred form of the electrode assembly the chamber is principally defined by two opposed barriers which are substantially vertical and planar, are spaced in substantially parallel relationship to each other and are spaced from the electrode. Side walls of the chamber may be of material impermeable to the electrolytic bath contents but the top and bottom of the chamber are preferably left open in order to permit the introduction and exit respectively of the electrolytic bath composition and the separated components of the introduced composition. In practice, under the influence of the electric current, a composition enriched in ionised material which accumulates in the region of one barrier surface is of different density from, and is frequently more dense than, a composition depleted in ionised material which accumulates at the-other barrier surface. Thus there is relative movement under the influence of gravity,-the composition enriched in ionised material mov ing towards the bottom of the chamber where it mixes with the main bath composition and the composition depleted in ionised material moving towards the top of the chamber where it maybe withdrawn. A proportion of the main bath composition is continually introduced into the chamber as a result of the circulatory movement taking place in the chamber.

It is desirable, for efficient operation, that the distance separating opposed barriers is substantially uniform. The electrodecantation process is improved, therefore, when the barriers which at least in part define the chamber are separated and maintained in position by spacers, for example of plastics material, of corrugated or similar structure. Whilst ensuring uniform separation of the opposed barriers, however, the process must also permit the ready separation of the compositions which accumulate in the region of the barrier surfaces. Preferably the spacers are perforated and define vertical channels for the relative movement of separated compositions. Suitable spacers are made of expanded diamond plastic mesh, vertically corrugated perforated mesh and certain other meshes (for example those obtainable commercially under the Registered Trade Mark Netlon).

In one form of the electrode assembly a chamber or chambers may be defined between a series of barriers which are separated by such spacers, the barrier and spacer elements having been inserted into a cage which is located within the outer casing of the assembly and positioned so that the barriers are suitably spaced from the electrode.

The process of electrodecantation described above is improved when the main composition of the electrolytic bath is introduced into two or more chambers which are situated in series with respect to the electrodes so that a single electric current can be passed through each of the chambers. In the electrode assembly described above two or more chambers may be defined, at least in part, between a series of barriers which comprise material substantially impermeable to ionised material but permeable to counterion and which are suitably spaced from the electrode.

The invention will now be described in more detail with respect to a process ofelectrodecantation wherein the ionised material is a coating material and conductive articles are concurrently coated by electrodepo sition. The invention is not limited by this description and those in the art will readily apply the following description to other electrolytic processes.

According to a further feature of the invention we provide a process of concentrating an electrolytic coating bath which contains a main liquid coating composition comprising ionised coating material, counterion and an aqueous medium, and concurrently coating a conductive article by electrodeposition, wherein an electric current is passed through the composition between the article and a counter electrode in order to deposit coating material on the article and at least a portion of the main composition is introduced into a chamber defined at least in part by two opposed barriers of material substantially impermeable to ionised coating material but permeable to counterion, said chamber being so situated in the bath that the electric current passes through the barriers and across the chamber, the components of the introduced composition which accumulate in the region of the barrier surfaces being separated and removed from the chamber and a composition enriched in ionised coating material returned to the main composition. Preferably the main composition is introduced into two or more chambers.

A composition enriched in ionised coating material accumulates in a chamber in the region of one barrier surface under the influence of the electric current and a composition depleted in ionised coating material but enriched in aqueous medium accumulates in the chamber'in the region of the other barrier surface. These compositions may be separated by any suitable means, for example under the influence of gravity when separation may be aided by the association of the ionised coating material with a material of density greater than either itself or the aqueous medium. Preferably the composition contains a pigment of density greater than the aqueous medium. The composition enriched in ionised coating material is returned to the main composition whilst the composition depleted in ioninsed coating material, but enriched in aqueous medium, is removed, thus effecting a concentration of the bath contents.

The composition enriched in aqueous medium may be discarded but since it may still contain useful coating composition constituents it may be further treated, for example by electrodecantation.

As already mentioned, it is very desirable to recover those constituents of coating compositions which are present in rinse water. Aqueous paint compositions contain expensive ingredients such as pigment and film-forming polymer and when very diluted with water, as in the case of rinse water, they can only beuti lised for coating after concentration. In the present process rinse water may be returned directly to the coating bath and concentrated therein, or optionally after concentration external of the bath.

Rinsing may be carried out with a composition which has been depleted in ionised material by electrodecantation and according to a further feature of the above process for concentrating an electrolytic coating bath containing a liquid coating composition comprising ionised coating material, counterion and aqueous medium and concurrently coating a conductive article by electrodeposition, a composition depleted in ionised coating material is withdrawn from the chamber and used to rinse an article which has been coated by electrodeposition and which carries a dip layer of coating composition.

The rinsings resulting from this process may be returned directly or indirectly, for example by way of a separate concentration stage, to the main composition in the electrolytic bath.

Preferably the counter electrode and the chamber or chambers in which the concentration of the coating composition is effected are comprised in one electrode assembly as described above. In one preferred em bodiment at least two such chambers are employed in conjunction with an electrode in an electrode assembly.

Although many ionised coating materials may be used, particularly suitable ionised coating materials for use in thisprocess are film-forming materials which contain acidic or basic groups and which are dispersible in an aqueous medium in the presence of a base or an acid respectively.

When referring to film-forming materials dispersible in an aqueous medium we mean that the ionised filmforming material is present in the aqueous medium in a stable form which may be for example in true, micellar or apparent solution as a colloidal dispersion or as an emulsion. Suitable film-forming materials contain ing acidic groups which, when ionised, constitute an ionised coating material in the present process include polycarboxylic resins, for example alkyd resins modified or unmodified with drying or non-drying oil residues, maleinised drying oils, esters of polyols with saturated or unsaturated fatty acids, epoxy resin esters with saturated or unsatruated fatty acids, and vinyl polymers which contain acid groups. Examples of alkyd resins are those based on trimellitic anhydride', examples of polyol esters, and including epoxy resin esters, are given in British Pat. No. 1,069,841; and examples of vinyl polymers include the copolymers of acrylic acid or methacrylic acid with monomers such as a hydroxy alkyl methacrylate and styrene. These materials may be dispersed in an aqueous medium in the presence of a base, for example a water soluble amine or an alkali metal hydroxide. Suitable water soluble amines include triethylamine, butylamine or diethanolamine and suitable alkali metal hydroxides include sodium and potassium hydroxide. The ionised base constitutes the counterion in the present process.

Suitable film-forming materials containing basic groups which when ionised constitute an ionised coating material in the present process include addition polymers which have been prepared by polymerising monomers at least one of which includes a basic group, for example a copolymer of diethylamino ethyl methacrylate. These materials may be dispersed in an aqueous medium in the presence of an acid, for example phosphoric acid. The ionised acid constitutes the counterion in the present process.

The precise nature of the opposed barriers which, at least in part, define each chamber and which are impermeable to ionised coating material but permeable to counterion, will be determined largely by the nature of the ionised coating material itself and that of the counterion. Those in the art will readily determine the type of barrier which is suitable to given conditions. Suitable barriers which are substantially impermeable to filmforming polymers containing acid or base groups such as, for example, the polycarboxylic acid resins mentioned above, are the conventional dialysis membranes of regenerated cellulose and cloth-like materials composed of natural and/or synthetic fibres. The pore size of such barriers will generally be in the region 100 300 A The use of a single barrier of this type has been described in a process of electrodepositing coatings for the purpose of controlling the pH of the coating bath and for restricting to the region of the counter electrode gas bubbles which arise thereon. In the present invention, however, there is associated with each electrode a series of at least two such barriers for the purpose of defining one or more chambers.

For tolerable pH control, it is a minimum requirement that a barrier of dialysis membrane is permeable to counterion but impermeable to ionised coating material. The control of pH in an electrodeposition coating bath id inefficient, however, when using conven tional dialysis membranes. Accurate control depends upon the permanent removal from the coating zone of an amount of counterion which is precisely equivalent to that amount of ionised coating material deposited on an article. With a conventional dialysis membrane barrier, however, counterion may return to the coating zone through the membrane when current is not passing to the coating zone. In British Pat. No. 1,106,979 we have described a process of pH control in a coating zone in which the counter electrode is separated from the article to be coated by an ion exchange membrane. The selectivity of such a membrane may be chosen to allow the unidirectional passage of counterion only under the influence of an electric current, thus preventing the return of counterion to the coating zone. Further, since an ion exchange membrane is substantially impermeable to water, the concentration process is more readily effected within each chamber.

Preferably in this invention a chamber is defined at least in part by one barrier which is comprised of an ion exchange membrane, and preferably the ion exchange membrane is of a pore size less than A. It is further preferred that the barrier adjacent the counter electrode is an ion exchange membrane, the counter electrode being in communication with the bath composilion only through such membrane. Preferably the counter electrode is enclosed by a casing having inlet and outlet ports to permit continuous or intermittent flushing of the region of the counter electrode. The

counter electrode is surrounded by aqueous medium, optionally containing dissolved electrolyte.

All of the barriers which define a chamber or chambers may be comprised of an ion exchange membrane and a preferred electrode assembly for use in the present processes comprises an electrode and two or more opposed barriers comprising ion exchange membrane which are spaced from the electrode and which barriers, at least in part, define one or more chambers.

One embodiment of the invention is now illustrated with reference to the accompanying drawings wherein FIG. 1 represents an electrolytic coating tank equipped to electrodeposit a coating onto an article,

FIG. 2 represents a partly sectioned perspective view of an electrode assembly suitable for use in the tank shown in FIG. 1, and

FIG. 3 shows the barrier-spacer arrangement.

Conductive articles may be coated separately or by a continuous process in an electrolytic bath containing a coating composition. With reference to the coating tank 1 in FIG. 1, a conductive article 3, which represents either an article to be coated separately or one of a series of articles to be coated continuously, is made the anode in an electric circuit and is immersed in a bath 2 of aqueous coating composition. The aqueous coating composition comprises a negatively charged coating material and a positively charged counterion. the counterion contributing to the stability of the ionised coating material in the bath. The ionised coating material may be, for example, an ionised polycarboxylic acid resin and the counterion a base ion. for example an alkali metal ion or an ionised amine. An electrode assembly 4, which contains an electrode 5 (the counter electrode), is made the cathode in the electric circuit referred to above and is placed in the coating bath so that the electrode is completely immersed.

With reference to FIG. 2, the electrode assembly consists of an outer casing 6 constructed at least in part of material suitable to impart rigidity thereto, for example plastics material, which is impermeable to and nonreactive with the bath contents. The electrode 5 is located in a substantially liquid tight chamber 7 bounded by outer impermeable walls of the casing and by an inner wall 8 of the assembly which is comprised of an ion exchange membrane. The chamber is filled with a simple aqueous electrolyte, and may be flushed either continuously or periodically through inlet and outlet ports 9 and 10. The electrode chamber 7 is separated by the wall 8 from a series of opposed barriers 14, also comprising ion exchange membrane, which are substantially vertical and spaced in parallel relationship to each other. The barriers are separated by perforated and corrugated spacers 15 of plastics material as illustrated in FIG. 3. Together with the side walls 11 of the assembly, to which they are attached, the opposed barriers define a series of chambers which are open at the top and at the bottom. The wall of the outer casing 6, which is remote from the electrode and which faces the article to be coated, also comprises an ion exchange membrane. Liquid may be withdrawn from a reservoir 12, above the series of chambers, by a suction device 13. The bottom of each chamber is in direct communication with the coating bath.

When an electric current is passed between the article 3 and the counter electrode 5, coating material deposits on the article and base counterion passes through the array of opposed barriers to be discharged at the counter electrode from whence it may be removed by flushing. Pigmented coating composition from the coating bath circulates upwards into the chambers formed between barriers 14 and under the influence of the electric current. There is an accumulation of a composition rich in pigment and ionised coating material at the barrier surfaces facing the counter electrodes and a corresponding accumulation at the reverse barrier surface of a composition depleted in pigment and ionised coating material. Due to the relative difference in densities, the enriched composition sinks to the bottom of the chamber and returns to the main bath composition whereas the depleted composition rises to the top of the chamber where it passes into a reservoir and can be removed therefrom by the suction device 13. Thus there is a net loss of the aqueous continuous phase from the coating bath. The removed composition may be, for example, further concentrated, used to rinse coated articles or discharged to waste.

We claim:

1. A process of concentrating an electrolytic coating bath which contains a main liquid coating composition comprising ionized coating material, counterion and an aqueous medium and concurrently coating a conductive article by electrodeposition, wherein an electric current is passed through the composition between the article and a counter electrode in order to deposit coating material on the article and at least a portion of the main composition is introduced into a chamber defined at least in part by two opposed barriers of material sub stantially impermeable to ionized coating material but permeable to counterion, said chamber being so situated in the bath that the electrodes are external to said chamber but the electric current passes through the barriers and across the chamber, the components of the introduced composition which accumulate in the region of the barrier surfaces being separated and removed from the chamber and a composition enriched in ionized coating material being returned to the main composition.

2. A process according to claim 1 wherein the main composition is introduced into two or more chambers.

3. A process according to claim 1 wherein at least one barrier is comprised of ion exchange membrane.

4. A process according to claim. 1 wherein the coating composition contains a pigment of density greater than the aqueous medium.

5. A process according to claim. 1 wherein the main liquid coating composition comprises a film-forming material containing acidic or basic groups which is dispersible in an aqueous medium in the presence of a base or an acid respectively.

6. A process according to claim 5 wherein the main liquid coating composition comprises a polycarboxylic acid resin.

7. A process for concentrating an electrolytic bath and concurrently coating a conductive article by electrodeposition according to claim 1. wherein a composition depleted in ionised coating material is withdrawn from the chamber or chambers and used to rinse an article which has been coated by electrodeposition and which carries a dip layer of coating composition.

8. A process according to claim 7 wherein the rinse product is returned directly or indirectly to the main composition in the electrolytic bath. 

1. A PROCESS OF CONCENTRATING AN ELECTROLYTIC COATING BATH WHICH CONTAINS A MAIN LIQUID COATING COMPOSITION COMPRISING IONIZED COATING MATERIAL, COUNTERION AND AN AQUEOUS MEDIUM AND CONCURRENTLY COATING A CONDUCTIVE ARTICLE BY ELECTRODEPOSITION, WHEREIN AN ELECTRIC CURRENT IS PASSED THROUGH THE COMPOSITION BETWEEN THE ARTICLE AND A COUNTER ELECTRODE IN ORDER TO DEPOSIT COATING MATERIAL ON THE ARTICLE AND AT LEAST A PORTION OF THE MAIN COMPOSITION IS INTRODUCED INTO A CHAMBER DEFINED AT LEAST IN PART BY TWO OPPOSED BARRIERS OF MATERIAL SUBSTANTIALLY IMPERMEABLE TO IONIZED COATING MATERIAL BUT PERMEABLE TO COUNTERION, SAID CHAMBER BEING SO SITUATED IN THE BATH THAT THE ELECTRODES ARE EXTERNAL TO SAID CHAMBER BUT THE ELECTRIC CURRENT PASSES THROUGH THE BARRIERS AND ACROSS THE CHAMBER, THE COMPONENTS OF THE INTRODUCED COMPOSITION WHICH ACCUMULATE IN THE REGION OF THE BARRIER SURFACES BEING SEPARATED AND REMOVED FROM THE CHAMBER AND A COMPOSITION ENRICHED IN IONIZED COATING MATERIAL BEING BEING RETURNED TO THE MAIN COMP SITION
 2. A process according to claim 1 wherein the main composition is introduced into two or more chambers.
 3. A process according to claim 1 wherein at least one barrier is comprised of ion exchange membrane.
 4. A process according to claim 1 wherein the coating composition contains a pigment of density greater than the aqueous medium.
 5. A process according to claim 1 wherein the main liquid coating composition comprises a film-forming material containing acidic or basic groups which is dispersible in an aqueous medium in the presence of a base or an acid respectively.
 6. A process according to claim 5 wherein the main liquid coating composition comprises a polycarboxylic acid resin.
 7. A process for concentrating an electrolytic bath and concurrently coating a conductive article by electrodeposition according to claim 1 wherein a composition depleted in ionised coating material is withdrawn from the chamber or chambers and used to rinse an article which has been coated by electrodeposition and which carries a dip layer of coating composition.
 8. A process according to claim 7 wherein the rinse product is returned directly or indirectly to the main composition in the electrolytic bath. 